starting problems 1989 500 SE

I have a 1989/90 Merecedes Benz 500 SE, and all the time since I purchased it the car has had a starting-problem. When it is cold or it has been stopped for 15 minutes or so, it takes up to 10 seconds before it fires properly enough to start. Then it starts on 6 or 7 cylinders and runs roughly for another 10-15 seconds before it starts to run smooth on all 8 cylinders. If I turn off the ignition when the engine has already started, and then try to start imediately, - vrrom!! it goes right away. The performance is 100% when it is already running, - there is only this starting-problem. Any ideas?

jaypee@organizer.net